1. Field of the Invention
The present invention relates to cooling of a multi-chip module. More precisely, the present invention relates to reducing the thermal resistance of a multi-chip module (i.e., "MCM") that uses flip chip and control collapse chip connection as the interconnect to the substrate in a cavity down ceramic package by adapting a heatspreader plate and a thermal shunt chip to the module.
2. Prior Art and Related Information
In conventional multi-chip modules, by packing a number of semiconductor devices in close proximity to each other while eliminating the individual packages for each of the devices, the electrical performance is improved and the board space occupied by the devices is reduced. Due to an increase in the packing density, however, the power density of the multi-chip module is typically higher than when separately packaged devices requiring more elaborate thermal design and thermal management schemes in order to maintain the device temperature within acceptable ranges are used.
In conventional multi-chip modules, the devices are connected to a substrate and the electrical connection among the devices is accomplished within the substrate, which may also be an integral part of the MCM package. One of the technologies used to connect the devices to the substrate is called flip chip and control collapse chip connection (i.e., "C4"). With this technology, solder bumps are developed at the chip terminals. Subsequently, the devices are flipped over on the substrate and the solder bumps are reflowed to make connection to the terminal pads on the substrate as shown in FIGS. 1(a) and 1(b). Internal thermal resistance and thermal performance of the MCM using flip chip and C4 interconnect technology are determined by the heat flow paths from the devices to the package body. Most of the heat generated by the devices flows out through one of the two primary heat flow paths in order to get to the package surface and eventually to a heat sink located on the package surface.
Based on the heat flow paths from the devices to the package body, the thermal design of modules with flip chip and C4 is categorized as a diverging design or a converging design. In a diverging design, the module may be arranged so that most of the heat flows out from the back side (passive side of the semiconductor device) as shown in FIG. 2(b). This arrangement is called a diverging MCM design because heat flows out of the device 1 in the opposite direction relative to the direction of the electrical signal flow, as indicated by the arrows in the magnified view of FIG. 2(a). More precisely, heat flows from the backside 8 of the device 1 through a thermally conductive medium 7 to the package body 4 and the heat sink 6 while the electrical signals flow out from the active side 9 of the device 1 to the substrate 3 and the interconnect layers within the substrate 3. Typically, the thermally conductive medium 7 is thermal grease or solder. A lid 5 encloses the devices 1 inside the package body 4.
In the case of a converging design, shown in FIGS. 3(a) and (b), the heat flows out from the active side 9' of the device 1', which is the same side as the circuitry. Hence, the electrical signals also flow out in the same direction as the heat flows. As indicated by the arrows in the magnified view of FIG. 3(a), the heat flows through the C4 solder bumps 2' into the substrate 3', from the substrate 3' to the package body 4', and eventually to the heat sink 6'. A lid 5' encloses the devices 1' inside the package body 4'.
Unfortunately, the conventional converging and diverging MCM designs have shortcomings. For example, the thermal performance of the converging multi-chip module with flip chip and C4 depends on thermal resistance through the solder bumps as well as the substrate's thermal characteristics. The thermal resistance from each device to the substrate is thus a function of the number and size of the bumps, and the thermal resistance of the substrate. But the number of the solder bumps is constrained by the number of electrical pads on the device as well as by the size of the available area on the die for placing electrically passive bumps (i.e., thermal bumps). Consequently, heat dissipation is limited by the area available for conduction.
Although the thermal resistance of the substrate can be decreased by incorporating thermal vias in the substrate, doing so increases the cost of the substrate and limits the routing capability of the substrate. Since heat management is a constant factor in circuit design, a need presently exists for effectively and efficiently cooling MCMs.